1. Technical Field
This invention relates to power actuators for servo operation of motor vehicle body closures. One common application of the invention will be in the form of powered actuators for remotely controlled locking and unlocking of vehicle passenger and driver's door latches e.g. as part of a central locking system; but the invention also extends to actuators for body closure of a vehicle other than the passenger or driver'doors, for example locking actuators attached to/or integrated into latch assemblies for vehicle boots or "hatchback" lids, sun roofs, bonnets and/or petrol or other filler lids or flaps; and/or to power actuators for movement or other operation of the closures themselves, for example, opening and closing vehicle windows and/or sunroofs.
2. Description of the Prior Art
There is an increasing demand for facilities and equipment on vehicles, even at the lower end of the volume production market, which provide ease of operation and added security, thus power actuators are required which are economical to manufacture and install, of simple construction, and reliable and durable in use. Limitations of the space available for installation, e.g. within vehicle doors and the desirability of avoiding unnecessary weight for greater vehicle efficiency gives rise to a demand for actuators which are compact and which utilise lightweight components even for their moving parts, for example moulded plastics gear wheels.
Due to the above factors the power unit most commonly employed in these actuators is a miniature rotary electrical motor operating at fairly high speed through a step-down gear train, commonly made up of lightweight plastics gear wheels, so as to provide the necessary torque and power output for reliable operation. Often the actuator mechanism includes provision for converting the rotary motion of the motor to linear motion of e.g. a push-pull plunger which is operatively linked to the part or parts of the body closure to be shifted, e.g. for locking and unlocking a door latch. Normally there is also provision for manual operation which commonly involves shifting the push-pull plunger with the associated drive gear train in a free-wheeling condition, i.e. on manual operation at lest some of the gear wheels in the train will be spun at relatively high speeds without carrying any substantial load.
It is most desirable that the rotating components should run freely both for power operation and in the manually induced "free-wheeling" mode for quiet and efficient operation and t avoid undue strain and wear and tear and the object of the invention is to provide a power actuator unit which meets the above requirements in a particularly simple and effective way without adding to its size, cost or complexity and which will ensure constant and efficient operation long term without servicing or maintenance and in the most adverse climatic conditions of heat or cold.
A problem which is prevalent and which has not hitherto been satisfactorily overcome in this type of actuator unit is the phenomenon hereinafter referred to as "racing" which will now be explained as follows:
Referring to FIG. 1 of the accompanying drawings a rotary drive component of a power actuator unit, for example a plastics gear wheel 10 is shown diagrammatically. The wheel has a central boss 12 defining a female bearing formation in the form of a cylindrical through bore 14 co-axial of the wheel.
Bore 14 is a running fit on a co-acting male bearing formation being a cylindrical metal tube shaft 16 fixed in a mounting being a body, casing or chassis (not shown) of the actuator unit.
Boss 12 may be regarded as an annulus having an internal diameter D riding on the shaft 16 which has an external diameter d which will be slightly less than D to provide the necessary running clearance (the difference in the diameter is shown greatly exaggerated in FIG. 1).
If wheel 10 is spun rapidly on shaft 16 particularly under free-wheeling no-load or very lightly loaded conditions there is a tendency for said annulus to ride round the shaft as if the latter was a toothed pinion meshed with an annular internally toothed gearwheel i.e. without slipping or sliding on the shaft periphery, the annulus swinging round the shaft int he manner of a "Hula-Hoop" causing a centrifugal force acting on a single contact point or line P which progresses round the shaft periphery.
When this "racing" effect takes place there is effectively an "harmonic drive" relating orbiting of the annulus to its swinging around the shaft by the formula ##EQU1## assuming that no sliding takes place at point P.
If, as will be the case where a shaft is a running fit in an annulus, D and d are close in size, the overall ratio is very high so that even if wheel 10, i.e., the annulus, is being driven for rotation at only moderately fast speeds, very high speed orbiting of the annulus can occur. The higher the speed of said orbiting, the greater the centrifugal force at the contact point P increasing the resistance to sliding and thus further ensuring continuance and build-up of the "racing".
The facing effect will be amplified if the rotating component such as gear wheel 19 is out of balance viewed in the axial direction along the shaft; such a condition is illustrated in FIG. 2 of the accompanying drawings where an annular boss 12a forms part of a bell-shaped component having a larger diameter portion 20 which projects axially from the boss and which is not directly supported or located on the shaft, its center of gravity (indicated at "C of G" on the drawing) being beyond the boss 12a.
Again, the out of balance effect is greatly exaggerated in FIG. 2, but it will be seen that the "racing" may take place with the non-slipping contact at very localised opposing positions A, B where the internal corners of the boss or annulus at its opposite ends engage opposite sides of the shaft periphery diagonally so that the component follows a conical envelope of revolution on the shaft with little or no slipping at said corner contact points.
The "racing" effect acts surprisingly powerfully to restrict or brake free rotation of the components on the shaft and causes unpleasant and noticeable vibrations accompanied by a whirring or buzzing noise which will often be amplified due the actuator unit being mounted within hollow portions of the vehicle body, such as the void within a door, and in contact, directly or indirectly, with metal door or other panels which may also resonate.
Some shapes of components are more susceptible to "racing" than others and in practice the presence or absence of the effect is found to be unpredictable. A batch of actuator units all made to the same design and tolerances may include some which operate quietly without "racing" and others in which the effect is so noticeable as to call for rejection. Hitherto, the only attempts made to avoid or mitigate this effect have been by manufacturing the components to extremely high tolerances and with highly polished and finished bearing surfaces so adding to manufacturing cost and quality control requirements; using specialised low friction materials, e.g. low friction plastics, which again adds to costs and may cause other problems as these materials may have disadvantages in other respects, e.g. as to durability, stability etc; and/or trying to ensure adequate and long term lubrication of the moving surfaces.
The latter expedient is most commonly employed but is not successful in practice, the choice of an appropriate lubricant is extremely difficult--a thick lubricant such as a grease may itself hinder effective operation of the actuator and will tend to deteriorate and become thicker with the passage of time, while a thin lubricant such as a light oil is quickly dispersed from the bearing surfaces due to their running pressures and "creep" as well as evaporation e.g. in hot conditions. Moreover the presence of lubricant can cause dust and dirt to collect on the bearing surfaces which will eventually cause excessive wear and increased friction. Motor vehicles have to operate under extremes of temperature and under winter conditions lubricant will tend to solidify and could even completely block operation of the actuator unit.